1. Field
This disclosure relates generally to gas delivery devices, and more particularly to a method of and system for fast pulse gas delivery. As used herein the term “gas” includes the term “vapors” should the two terms be consider different.
2. Overview
The manufacture or fabrication of semiconductor devices often requires the careful synchronization and precisely measured delivery of as many as a dozen gases to a process tool such as a vacuum processing chamber. Various recipes are used in the manufacturing process, and many discrete processing steps, where a semiconductor device is cleaned, polished, oxidized, masked, etched, doped, metalized, etc., can be required. The steps used, their particular sequence, and the materials involved all contribute to the making of particular devices.
As more device sizes have shrunk below 90 nm, atomic layer deposition, or ALD processes continues to be required for a variety of applications, such as the deposition of barriers for copper interconnects, the creation of tungsten nucleation layers, and the production of highly conducting dielectrics. In the ALD process, two or more precursor gases are delivered in pulses and flow over a wafer surface in a process chamber maintained under vacuum. The two or more precursor gases flow in an alternating or sequential manner so that the gases can react with the sites or functional groups on the wafer surface. When all of the available sites are saturated from one of the precursor gases (e.g., gas A), the reaction stops and a purge gas is used to purge the excess precursor molecules from the process chamber. The process is repeated, as the next precursor gas (i.e., gas B) flows over the wafer surface. For a process involving two precursor gases, a cycle can be defined as one pulse of precursor A, purge, one pulse of precursor B, and purge. A cycle can include the pulses of additional precursor gases, as well as repeats of a precursor gas, with the use of a purge gas in between successive pulses of precursor gases. This sequence is repeated until the final thickness is reached. These sequential, self-limiting surface reactions result in one monolayer of deposited film per cycle.
The pulses of precursor gases into the processing chamber are normally controlled using on/off-type valves which are simply opened for a predetermined period of time to deliver a desired amount (mass) of precursor gas with each pulse into the processing chamber. Alternatively, a mass flow controller, which is a self-contained device comprising a transducer, control valve, and control and signal-processing electronics, is used to deliver an amount of gas (mass) at predetermined and repeatable flow rates, in short time intervals. In both cases, the amount of material (mass) flowing into the process chamber is not actually measured, but inferred from measuring parameters of the ideal gas law.
Systems known as pulse gas delivery (PGD) devices have been developed that can measure and deliver pulsed mass flow of precursor gases into semiconductor processing chambers and other processing tools. Such devices are designed to provide repeatable and precise quantities (mass) of gases for use in semiconductor manufacturing processes, such as atomic layer deposition (ALD) processes.
PGDs usually include a delivery reservoir or chamber containing the gas to be delivered during the ALD process upstream to the processing chamber or tool. By measuring the pressure and temperature of the gas in the delivery chamber, and controlling the flow of gas from the delivery chamber as a function of the pressure drop of the gas in the chamber during delivery, the mass of a pulse of gas delivered during the ALD can be precisely controlled. The flow of the pulse of gas from the chamber is controlled with an on/off-type outlet valve between the delivery chamber of the PGD and the process tool receiving the gas. The amount of time the valve is required to be open to deliver a pulse of gas of a given mass is a further function of the starting pressures of the gas in the chamber and the downstream pressure of the processing chamber or tool. For example, for a given amount of gas that needs to be delivered, the starting pressure in the delivery chamber at a higher starting pressure requires a shorter time for the valve to be open than at a lower starting pressure since the mass flow occurs more quickly at the higher starting pressure. The charge period and the delivery period of PGDs should be tightly controlled for fast pulse gas delivery applications in order to insure accurate delivery of prescribed amounts of gas(es). As a result, the upstream pressure of the PGDs as well as the charged pressure in the PGDs should be tightly controlled in order to meet the repeatability and the accuracy requirement of the ALD process.
Further, the inlet and outlet valves in the PGD have a finite response time to transition from one state (on/off) to another state (off/on) when the valves are commanded for either charging the chamber or delivering the gas pulse. For example, a typical response time of pneumatic shut-off valves in ALD applications is between about 5 and 35 milliseconds. The response time of the valves can introduce a delay to a response to the valve command sent by the PGD controller, which causes either an overcharging of the PGD chamber or overdelivering of the gas pulse to the processing chambers or tools as illustrated in FIG. 2. For example, in the charging mode of the PGD, the outlet valve is shut, the inlet valve is open for gas to enter the delivery chamber of the PGD, and the PGD controller monitors the pressure change. The PGD controller needs to send a shut-off command to the inlet valve early before the delivery chamber reaches the pressure setpoint by considering the response time (or delay) of the inlet valve; otherwise, the delivery chamber can be overcharged or the delivery chamber pressure is above the setpoint.
More recently, certain processes have recently been developed that require high speed pulsed or time-multiplexed processing. For example, the semiconductor industry is developing advanced, 3-D integrated circuits thru-silicon vias (TSVs) to provide interconnect capability for die-to-die and wafer-to-wafer stacking. Manufacturers are currently considering a wide variety of 3-D integration schemes that present an equally broad range of TSV etch requirements. Plasma etch technology such as the Bosch process, which has been used extensively for deep silicon etching in memory devices and MEMS production, is well suited for TSV creation. The Bosch process, also known as a high speed pulsed or time-multiplexed etching, alternates repeatedly between two modes to achieve nearly vertical structures using SF6 and the deposition of a chemically inert passivation layer using C4F8. Targets for TSV required for commercial success are: adequate functionality, low cost and proven reliability.
Currently, there are two prior art approaches for high speed pulse gas delivery in a Bosch process. The first prior art approach is to use fast response mass flow controllers (MFCs) to turn on and off gas flows of the delivery pulse gases. This method suffers from slow delivery speed, and poor repeatability and accuracy. The second prior art approach involves using MFCs coupled with downstream three-way valves. The MFCs maintain constant flow and the downstream three-way valves switch between the process line and the divert dump line frequently in order to deliver pulse gases to the process chamber. Clearly, a lot of gases are wasted, which increases the process cost. The second method also suffers from repeatability and accuracy of delivery. Thus, it is desirable to provide a solution for high speed pulse delivery applications, such as the Bosch process used for TSV creation, that reduce or overcome these problems.